112 



VERTEBRATE SKELETON 



parallel with the premaxilla, the latter alone forming the functional 

 margin of the .jaw, and, except in some Acanthini, alone bearing 

 teeth (lacking on the premaxilla of some eels). In these forms the 

 upper jaw, and especially the premaxilla, rests on a cartilage (prob- 

 able homologue of the first upper labial) and has great mobility, 

 the premaxilla often having an ascending process connected with the 

 cranium by an elastic ligament. In some Teleosts the mobility is 

 largely or wholly lost, the premaxilla being firmly attached to the 

 ventral side of the supraethmoid. The premaxilla is greatly elongate 



Fig. 117. — Skull of Scomber (AUis, '03). ar, articular; d, dentale; en, entopterygoid; 

 e, exoccipital; ep, ectopterygoid; et, ethmoid; /, frontal; hm, hyomandibula; io, inter- 

 operculum; I, lacrimal; rn, maxilla; mp, inetapterygoid; n, nasal; op, operculare; p, 

 parietal; pe, petrosal; pf, postfrontal; pi, palatine; pm, premaxilla; pop, preoperculum; 

 po, postorbitals; prf, prefrontal; ps, parasphenoid; soc, supraoccipital; so, suborbital; 

 sop, suboperculum; sp, sphenotic; sq, squamosal; sy, symplectic. 



in a few fishes {Belone, Xiphias) and, with the vomer and maxilla, 

 forms the sword of swordfish. Maxilla and premaxilla are fused in 

 Gymnodonts, and in Diodon and Mormyrus the premaxillas of the 

 two sides are united. 



The membrane bones of the oral roof are much as outlined on p. 

 94; the pterygoids were noticed above. The thin and elongate 

 parasphenoid extends from the basioccipital which it overlaps, to 

 the vomer. Its relations to the eye-muscle canal were mentioned 

 above. The vomer of the adult is broader in front and tapers back- 

 wards. In Amiurus it arises from a single centre, from a pair in 



